Sorghum is the second most important cereal-feed grain grown in the United States. Production is economically critical to farms operating in marginal rainfall areas because of sorghum's ability to tolerate drought and heat. Both the livestock and bio-energy industries utilize sorghum as an energy substrate thereby making it a versatile crop.
Worldwide, sorghum is the fifth leading cereal grain. As it is tolerant to both drought and heat, it is easily the most widely grown food grain in the semiarid regions of sub-Sahelian Africa and in the dry central peninsular region of India. As such, sorghum is used in human consumption in most of the driest regions of the world thereby making it a critically important food crop in these locations.
The development of herbicide resistance in plants offers significant production and economic advantages, as such the use of herbicides for controlling weeds in crops has become almost a universal practice. However, application of such herbicides can also result in death or reduced growth of the desired crop plant, making the time and method of herbicide application critical or in some cases unfeasible.
Of particular interest to farmers is the use of herbicides with greater potency, broad weed spectrum effectiveness and rapid soil degradation. Plants, plant tissues and seeds with resistance to these compounds provide an attractive solution by allowing the herbicides to be used to control weed growth, with small risk of damage to the crop. One such class of broad-spectrum herbicides are those that inhibit the activity of the acetolactate synthase (ALS) enzyme in a plant. Acetolactate synthase is required for the production of essential amino acids such as valine, leucine and isoleucine in plants (this biochemical pathway is not present in humans or other animals). Sorghum is susceptible to many ALS inhibiting herbicides that target monocot species, making the use of these herbicides to control grassy weeds almost impossible, as they will also inhibit the growth of the crop plant.
Acetolactate synthase herbicides control a wide spectrum of grass and broadleaf weeds at very low application rates. There are currently over 56 different ALS herbicides available, made from various formulations of sulfonylureas (SU), imidazolinones (IMI), triazolopyrimidines (TP) and pyrimidinylthiobenzoates (PTB). Mutations in some crop plants, for example tobacco, corn (U.S. Pat. No. 5,767,361) and soybeans, have been found that confer ALS herbicide resistance, however, to date no such discovery has been found in sorghum (for a review, see Tan et al., 2005, Pest. Manag. Sci. 61:246-257). Due to the importance of sorghum on the world stage, what are needed are sorghum cultivars that are resistant to the inhibitory effects of ALS herbicides, thereby allowing for greater crop yield when these herbicides are used to control grassy weeds.